Download Igneous minerals

Igneous
Mineral
Associations
How we know the mineralogy of the mantle:
• Seismic wave velocities indicate rocks are either peridotite or eclogite
• Natural samples of the mantle: xenoliths in lava, rocks from tectonically disrupted
lithosphere (ophiolites) are mostly peridotite
• Mantle rocks must melt to form basaltic magma
3 types of primary basaltic magma
•
•
•
At divergent plate margins (mid ocean ridges) – magma rises from asthenosphere decompression melting at low pressure - tholeiitic basaltic magma
At hot spot (intra-plate volcanoes) – magma rises from deep mantle - decompression melting at
high pressure - alkalic basaltic magma
At convergent plate margins (volcanic arcs) – water added to the mantle from the subducted
lithosphere causes melting - flux melting - calc-alkaline or high alumina basaltic magma
Mineral association 1)
Peridotite contains
olivine + clinopyroxene
+ orthopyroxene and it
melts partially to form
basaltic magma
Lherzolite – harzburgite – dunite
sequence is formed by magma extraction.
Dunite is >90% olivine.
Phase diagram for Lherzolite
Si [4] => Si [6]
After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
Intrusive and extrusive igneous rocks
Intrusive (plutonic)
igneous rocks form
from magma that
cools beneath the
earth’s surface
Extrusive (volcanic)
igneous rocks form
from magma that
cools at the earth’s
surface:
Lava
Pyroclastics
Fine grained
Volcanic or
extrusive rocks
Coarse grained
Plutonic or
intrusive rocks
TEXTURE
COMPOSITION
Mafic rocks:
Basalt and
gabbro –
contain mafic
minerals
Felsic rocks:
Rhyolite and
granite – contain
feldspar and
quartz
The common igneous rocks are classified on the basis of: SiO2 and the
percentage of mafic minerals, in addition to texture.
Most magma entering the Earth’s crust
from the mantle is basaltic (about 50%
SiO2, high Mg, Fe, Ca, low Na, K)
How does intermediate and felsic magma
form?
Why are there different types of magma ?
Experiments on molten rocks showed that there was a sequence in
which minerals crystallized from basaltic magma as the magma cooled
Minerals such as plagioclase feldspar and olivine also change
composition as the magma cools
Albite =
NaAlSi3O8
Anorthite =
CaAl2Si2O8
Magmatic differentiation by fractional crystallization
The liquid part of magma changes
composition because minerals with high
crystallization temperatures settle out or
accumulate at magma chamber
boundaries. The elements in these solid
minerals are removed from the magma, so
that it changes composition, usually
toward higher SiO2, lower Mg, Fe and Ca,
higher Na, K.
Other magmatic differentiation
processes:
-assimilation of crustal rocks – plenty of
geochemical evidence for this
-magma mixing
Magmatic differentiation
Dissolved gases also exsolve as the
magma solidifies. Elements that are
not accommodated in the common
rock forming minerals are also
concentrated in the last remaining
liquid/gas-charged magma. These
late-stage magmas form pegmatites.
Pegmatites
Layered Mafic Intrusions
Some Igneous Mineral Associations:
1) Peridotites (mantle rocks)
2) Layered mafic intrusions
3) Pegmatites
4) Kimberlites
http://www.mineralsed.ca/i/video/kimberlite-anim.swf
5) Carbonatites
Also – the common igneous rocks